There a number of different systems designed to purify wastewater produced by industry and municipal sources. Activated sludge wastewater treatment plants (WWTPs) are a type of biological reactor (bioreactor) system which are commonly used to treat industrial and municipal wastewater.
Various activated sludge processes are currently used in wastewater treatment plants to facilitate degradation of contaminants. In most wastewater treatment plants employing activated sludge either suspensions of activated sludge or fixed films of activated sludge are used to facilitate degradation of contaminants. Typically, activated sludge processes use aerobic, anoxic and anaerobic zones depending upon the desired level of treatment. The aerobic, anoxic and anaerobic zones perform important functions in the treatment process.
In the aerobic zones present in the activated sludge process stream, oxygen is often supplied by sparging air into the wastewater stream at an airflow rate sufficient to maintain a given dissolved oxygen level. In the aerobic zone, nitrifying autotrophic microbes capable of using NH4+ (ammonia) as their energy source convert ammonia to nitrite then to nitrate and heterotrophic microbes consume available carbon in the water phase.
Anoxic zones are also present in the activated sludge process stream. Such anoxic zones lack dissolved oxygen. In the anoxic zones, denitrifying heterotrophic microbes utilize nitrate and nitrite as electron acceptors and can reduce the amount of nitrogen present in the water. Denitrifying heterotrophic microbes also consume some of the available carbon during the denitrification process. During denitrification, “NO,” species are reduced stepwise from NO3 (nitrate)→NO2 (nitrite)→NO (nitric oxide)→N2O (nitrous oxide)→N2 to nitrogen gas which is ultimately released to the atmosphere. In a typical activated sludge process, nitrate for denitrification is often supplied by providing wastewater to the beginning of the anoxic stage.
Anaerobic zones are also present in the activated sludge process stream. Anaerobic zones lack dissolved oxygen, nitrate and nitrite. In the anaerobic zones, a portion of an available carbon source in the wastewater is removed by polyphosphate forming microorganisms during their growth and, in turn, inorganic phosphate (PO4−) is released into water. This rapid uptake and storage of available carbon by polyphosphate forming microorganisms in the anaerobic zones insures phosphate removal later in the anoxic and aerobic zones of the process stream.
Importantly, the amount of airflow to maintain a given amount of dissolved oxygen in the aerobic zones of the biological reactor is not constant. This is because the load of organic nitrogen and carbon compounds in the materials in the wastewater stream entering the activated sludge process stream is not constant. For example, there are diurnal fluctuations in the municipal wastewater stream such that at night the loading levels are lower, but during the daytime loading levels are higher. In fact, clear hourly trends in municipal wastewater loading levels can be seen which correlate to municipal water usage during waking hours (e.g. shower and other bathroom usage). Similarly, there are seasonal fluctuations in the amount of dissolved oxygen available which is, in part, a function of water temperature as well as decreased microbial activity at lower temperatures and increased microbial activity at high temperatures as well as a number of other variables.
Altogether, this means that the dynamics of a typical wastewater treatment system are non-linear and vary over time. These fluctuations may result in insufficient control of the dissolved oxygen (DO) concentration in the aerobic zones depending on the circumstances.
A number of different parameters of activated sludge wastewater treatment processes can be monitored. These parameters include biological oxygen demand (BOD), chemical oxygen demand (COD), nitrogen levels measured as total Kjeldahl nitrogen (TKN), nitrate levels (NO3), nitrite levels (NO2), phosphorous levels measured as inorganic phosphate (PO4−) as well as other parameters such as total suspended solids, temperature, and pH. A number of these parameters such as airflow rates, dissolved oxygen concentrations, medium flow rates, mixed liquor suspended solids concentrations, NH4+ concentrations and temperature can be monitored in real-time using automated instrumentation
Aeration control systems provide a supply of oxygen in aerobic zones to meet constantly changing oxygen demands of wastewater treatment systems. A typical aeration system contains blowers and air control valves to regulate airflow rates and the sparging of air to maintain a given dissolved oxygen level. However, oxygen can also be provided by means of other gases (e.g. compressed gases such as pure oxygen). Conventional aeration control systems use readings from a dissolved oxygen sensor with a proportional-integral controller to determine the desired change in airflow needed from the aeration system to restore the predetermined, user-selected dissolved oxygen set point when deviations from this set point occur (e.g. airflow is increased after too little dissolved oxygen is sensed). FIG. 1 depicts a conventional bioreactor based wastewater treatment system and conventional aeration control system.
Although a large spectrum of activated sludge wastewater treatment process parameters can be monitored to provide a wealth of information, effectively utilizing this information to maximize activated sludge process efficiency presents a difficult problem. This means that conventional techniques of aeration control result in unnecessarily high energy consumption and poor wastewater treatment efficiency. Thus, a need exists for methods of selecting airflow rates to maintain a desired dissolved oxygen level in the aerobic zone of a wastewater treatment process stream such that excessive energy is not consumed and high treatment efficiency is maintained. In other words, there is a need for methods of providing an aerobic medium in a bioreactor compartment and systems that perform such methods.